1. Field of the Invention
The present invention relates to a semiconductor device using a semiconductor element.
2. Description of the Related Art
In general, the output of an analog circuit is required to be highly accurate and stable regardless of fluctuation in power supply voltage or the like, and thus stable reference voltage is needed in many cases. A reference voltage generating circuit for generating such stable reference voltage is known. As reference voltage generating circuits, a variety of circuits of a threshold voltage-based type, a β multiplier self-bias type, a bandgap-based type, and the like are known. Most of these reference voltage generating circuits are driven by being supplied with slight current.
However, when power supply voltage is simply input to such a reference voltage generating circuit at the time of startup thereof, the reference voltage generating circuit is not started up properly in some cases. Specifically, in some cases, even when power supply voltage is input, the reference voltage generating circuit still exits in a stable state where current does not flow in the circuit, and therefore the circuit is not started up or it takes a significantly long time for the circuit to reach a stable equilibrium state even if the circuit is started up. Thus, a method is known in which a startup circuit for applying initial voltage that prompts startup of a reference voltage generating circuit when power is input is connected thereto in order to achieve quick startup of the reference voltage generating circuit (Non-Patent Document 1).
FIG. 8 illustrates an example of a configuration of a β multiplier self-bias reference voltage generating circuit to which a conventional startup circuit is connected. A startup circuit 501 includes a transistor 511, a transistor 512, and a transistor 513. A first electrode of the transistor 511 is connected to a power input portion VDD, and a second electrode and a gate of the transistor 511 are connected to a first electrode of the transistor 512 and a gate of the transistor 513. A gate of the transistor 512 is connected to a second electrode of the transistor 513, and a second electrode of the transistor 512 is connected to a ground voltage input portion GND. Here, a node connected to the gate of the transistor 511 is referred to as a node (a).
A reference voltage generating circuit 502 includes a transistor 521, a transistor 522, a transistor 523, a transistor 524, and a resistor 525. A first electrode of the transistor 521 is connected to the power input portion VDD, and a gate and a second electrode of the transistor 521 are connected to a gate of the transistor 522 and a first electrode of the transistor 523. A first electrode of the transistor 522 is connected to the power input portion VDD, and a second electrode of the transistor 522 is connected to a gate of the transistor 523 and a first electrode and a gate of the transistor 524. A second electrode of the transistor 523 is connected to a first electrode of the resistor 525. A second electrode of the resistor 525 and a second electrode of the transistor 524 are connected to the ground voltage input portion GND. Here, a node connected to the gates of the transistor 521 and the transistor 522 is referred to as a node (b), and a node connected to the gates of the transistor 523 and the transistor 524 is referred to as a node (c). Note that the node (c) corresponds to a node of an output portion OUT.
A first electrode and the second electrode of the transistor 513 in the startup circuit are connected to the node (b) and the node (c), respectively, so that the startup circuit 501 and the reference voltage generating circuit 502 are electrically connected to each other.
Note that the transistor 511, the transistor 521, and the transistor 522 are each a p-channel transistor, and the transistor 512, the transistor 513, the transistor 523, and the transistor 524 are each an n-channel transistor. In this configuration, a load capacitor 531 is connected as an output load.
Power supply voltage Vdd is applied to the power input portion VDD. When power is not supplied, the power input portion VDD can be in a floating state or supplied with ground voltage Vgnd. Further, the ground voltage Vgnd is input to the ground voltage input portion GND. Here, voltage lower than the power supply voltage Vdd can be used instead of the ground voltage Vgnd. For example, common voltage which is common to the circuits or 0 V can be used as the ground voltage Vgnd. The ground voltage input portion GND may be set at low power supply voltage. Note that the low power supply voltage is voltage which is lower than high power supply voltage when the high power supply voltage used for the power input portion VDD is a reference. In this configuration, the ground voltage Vgnd is applied to the ground voltage input portion GND.
Next, operation of the startup circuit 501 and the reference voltage generating circuit 502 will be described.
First, before power is input, that is, when the power supply voltage Vdd is not applied to the power input portion VDD, current does not flow into the transistor 521, the transistor 522, the transistor 523, the transistor 524, and the resistor 525 in the reference voltage generating circuit 502. Here, the state where all the transistors are in an off state and current does not flow thereinto is one of metastable states of the reference voltage generating circuit 502.
Next, the power supply voltage Vdd is applied to the power input portion VDD. However, since the reference voltage generating circuit 502 is stabilized in the metastable state, the reference voltage generating circuit 502 operates to keep this state even when the power supply voltage Vdd is applied to the power input portion VDD. In other words, right after the power supply voltage Vdd is applied to the power input portion VDD, the voltage of the node (b) connected to the gates of the transistor 521 and the transistor 522 becomes Vdd so that a voltage difference is not generated between the gate and the source of each of the transistor 521 and the transistor 522, whereby the off state of the transistors is maintained. In a similar manner, the voltage of the node (c) connected to the gates of the transistor 523 and the transistor 524 becomes the ground voltage Vgnd so that the transistor 523 and the transistor 524 are in the off state.
Meanwhile, in the startup circuit 501, when the power supply voltage Vdd is applied to the power input portion VDD, the voltage of the node (a) connected to the gate of the transistor 511 is changed from Vdd to voltage between Vdd and Vthp (here, Vthp is the threshold voltage of each p-channel transistor). Thus, a voltage difference is generated between the gate of transistor 513 connected to the node (a) and the second electrode thereof, the transistor 513 is turned on, and current flows from the node (b) toward the node (c). Accordingly, the voltage of the first electrode of the transistor 513, that is, the voltage of the node (b) drops from the power supply voltage Vdd; at the same time, the voltage of the second electrode of the transistor 513, that is, the voltage of the node (c) rises from the ground voltage Vgnd.
In the reference voltage generating circuit 502, the voltage of the node (b) drops from Vdd, which allows the transistor 521 and the transistor 522 to be turned on; at the same time, the voltage of the node (c) rises from the ground voltage Vgnd, which allows the transistor 523 and the transistor 524 to be turned on. Consequently, the reference voltage generating circuit 502 leaves from the metastable state where current does not flow and starts operating.
On the other hand, a rise in the voltage of the node (c) allows the transistor 512 whose gate is connected to the node (c) to be turned on. Accordingly, current flows through the transistor 512 so that the voltage of the node (a) drops to the ground voltage Vgnd and thus the transistor 513 is turned off. When the transistor 513 is turned off, current flowing from the node (b) to the node (c) as described above is blocked and the startup circuit 501 is completely electrically isolated from the reference voltage generating circuit 502.
After that, the reference voltage generating circuit 502 reaches a stable equilibrium state. In other words, the voltage of the node (b) drops from Vdd and then reaches to certain voltage higher than or equal to the ground voltage Vgnd and lower than or equal to the power supply voltage Vdd and is stabilized; in a similar manner, the voltage of the node (c) rises from the ground voltage Vgnd and then reaches certain voltage higher than or equal to the ground voltage Vgnd and lower than or equal to the power supply voltage Vdd and is stabilized. Here, the voltage of the node (c) corresponds to output voltage of the reference voltage generating circuit 502.
In this manner, when power is input, the startup circuit functions to input voltage which allows the reference voltage generating circuit to leave from a metastable state and prompts startup thereof to the reference voltage generating circuit.